Two-dimensional hydrogen-bonded polymers in the crystal structures of the ammonium salts of phenoxyacetic acid, (4-fluorophenoxy)acetic acid and (4-chloro-2-methylphenoxy)acetic acid

The crystal of the isomorphous anhydrous ammonium salts of phenoxyacetic acid and (4-fluorophenoxy)acetic acid and that of the hemihydrate ammonium salt of 4-chloro-2-methylphenoxy)acetic acid show two-dimensional layered structures based on conjoined cyclic hydrogen-bonded motifs.

Noring associations are present in any of the structures.

Chemical context
The crystal structures of the ammonium salts of carboxylic acids are, despite their simple formulae, characterized by the presence of a complex array of hydrogen-bonding interactions. From a study of the packing motifs of the these ammonium carboxylate salts from examples in the Cambridge Structural Database (Groom & Allen, 2014), Odendal et al. (2010) found that two-dimensional hydrogen-bonded nets, ladders or cubane-type structures could be predicted on the basis of the size and conformation of the anions. These structures are often stabilized byaromatic ring interactions. With the benzoic acid analogues, two-dimensional sheet structures are common with interactions involving the ammonium cations and the carboxylate anions in N-HÁ Á ÁO hydrogen bonding, forming core layer structures, with the aromatic rings occupying the interstitial cell regions, e.g. with benzoic acid (Odendal et al., 2010), 3-nitrobenzoic acid (Eppel & Bernstein, 2009) and 2,4-dichlorobenzoic acid (Smith, 2014). Three-dimensional structures are usually only formed when interactive substituent groups are present on the benzoate rings, interlinking the layers e.g. with 3,5-dinitrobenzoic acid (Smith, 2014). The presence of water molecules of solvation may also produce a similar effect, although these are usually confined to the primary cation-anion layers.

Supramolecular features
In the crystals of (I) and (II), two H atoms of the ammonium group give cyclic asymmetric three-centre (bifurcated) N-HÁ Á Á(O,O) hydrogen-bonding interactions with the anion (Tables 1 and 2, respectively). One of these is with two O-atom acceptors of the carboxyl group (O13, O14) [graph set R 2 1 (4)], the other is with the carboxyl and phenoxy O-atom acceptors (O13 ii , O11 ii ) of an inversion-related anion [graph set R 2 1 (5)]. These, together with a third N1-H13Á Á ÁO13 ii hydrogen bond, give a cyclic R 2 4 (8) ring motif, forming a series of conjoined rings which extend the structures along c. The other H atom gives structure extension through an N-HÁ Á ÁO hydrogen bond to a carboxyl O atom (O14 iii ), forming a two-dimensional sheet-like structure which lies parallel to (100). Present in the crystal are short inversion-related intermolecular F4Á Á ÁF4 iv contacts of 2.793 (2) Å [symmetry code: (iv) Àx + 2, Molecular conformation and atom labelling for (I), with inter-species hydrogen bonds shown as a dashed lines (see Table 1 for details). Non-H atoms are shown as 40% probability displacement ellipsoids.

Figure 2
Molecular conformation and atom labelling for (II), with inter-species hydrogen bonds shown as dashed lines (see Table 2 for details). Non-H atoms are shown as 40% probability displacement ellipsoids.

Figure 3
Molecular conformation and atom labelling for (III), with inter-species hydrogen bonds shown as dashed lines (see Table 3 for details). Non-H atoms are shown as 40% probability displacement ellipsoids.

Figure 4
The two-dimensional hydrogen-bonded network structure of (I), which is equivalent to that of the isomorphous compound (II). Hydrogen bonds are shown as dashed lines and non-associative H-atoms have been omitted [for symmetry codes see Tables 1 and 2].

Figure 6
The two-dimensional hydrogen-bonded network structure of (III) in the unit cell, viewed along b.

Synthesis and crystallization
The title compounds were prepared by the addition of excess 5 M aqueous ammonia solution to 1 mmol of either phenoxyacetic acid [150 mg for (I)], (4-fluorophenoxy)acetic acid [170 mg for (II)] or (4-chloro-2-methylphenoxy)acetic acid [200 mg for (III)] in 10 mL of 10% ethanol-water. Roomtemperature evaporation of the solvent gave colourless platelike crystals of (I), (II) and (III) from which specimens were cleaved for the X-ray analyses.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 4. Hydrogen atoms potentially involved in hydrogen-bonding interactions were located in difference Fourier maps but were subsequently included in the refinements with positional parameters fixed and with U iso (H) = 1.2U eq (N) or = 1.5U eq (O). Other H atoms were included at calculated positions [C-H(aromatic) = 0.95, C-H(methylene) = 0.98, C-H(methyl) = 0.97 Å ] and also treated as riding, with U iso (H) = 1.5U eq (C) for methyl H atoms and = 1.2U eq (C) for other H atoms. In (III), the methyl group was found to be rotationally disordered, with the H atoms distributed over six equivalent half-sites, and was treated accordingly.   (2), 7.1453 (6), 6.7243 (7) 18.386 (2), 7.1223 (6), 6.7609 (6) 38.0396 (9)  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.29 e Å −3 Δρ min = −0.24 e Å −3 Special details Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq O11 0.71681 (10) 0.4894 (2) −0.0073 (2) 0.0357 (6)   Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq F4 0.97409 (8) 0.4959 (2) −0.3100 (2) 0.0761 (6)   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.32 e Å −3 Δρ min = −0.28 e Å −3 Special details Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All su's are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.